The field of the disclosure relates generally to stick-slip detection, more particularly, to systems and methods for detecting an occurrence of a stick-slip by performing online diagnostics for a valve.
Stick-slip in a control valve refers to an inability to control a position of the valve due to high static friction and low kinetic friction between a valve stem and valve packing. The stick-slip phenomenon occurs when an attempt is made to move a position of the valve to a setpoint. However, when a force is applied to move the valve, the force does not immediately overcome the static friction. Therefore, the force is increased until the static friction is overcome, which results in a sudden jump due to low kinetic friction. Unfortunately, the force required to overcome the static friction forces the valve to overshoot a desired setpoint and, as a result, the valve is stuck at a new undesirable position. This process is repeated in the opposite direction resulting in cycling of the valve back and forth across desired setpoints in an undesirable manner that is not well-controlled. As a result of stick-slip cycling, the valve wears out faster and the process being controlled by the valve is negatively affected. However, if it is known that a control valve is undergoing stick-slip, then adjustments may be made or maintenance performed to correct the problem.
Existing systems detect stick-slip by performing periodic diagnostics on control valves to determine operability and performance of the valves. These existing systems use various sensors and other measurement devices, such as a digital valve controller, to measure and collect data from various sensors on a control valve. However, determining whether a stick-slip has occurred is typically unknown in these existing systems until the collected data is reviewed by an operator who then determines if the collected data demonstrates characteristics of an occurrence of a stick-slip. As a result, existing systems prove to be very inefficient in stick-slip detection as a detection of a stick-slip cannot be achieved very quickly or very reliably if the data is analyzed offline and if characteristics of stick-slip are similar to other system errors. This not only results in inadequate timing of stick-slip detection, but also results in a misdiagnosis of stick-slip detection.